Did you know that the Appalachian Mountains can affect weather and climate a long distance away from where they are? The cold air damming that we call “The Wedge” is one way that it affects local conditions here in Athens GA and in places as far away as Birmingham. Downslope winds when the atmosphere is bringing us air flow from the northwest is another way. We saw an example of that today when we were driving back from Birmingham AL to Athens GA. Here is a Facebook post from my meteorologist husband John Knox on what we saw:
Driving back to Athens today, we got to see a pretty clear-cut example of downsloping -> cloud dissipation. We were in low stratocumulus for most of the drive, but at Conyers I could see the edge of the cloud deck. Sure enough, about 30 miles from Athens the low clouds parted, and it was sunny as we pulled into town.
Why? For the same reason that thunderstorms coming toward Athens from the northwest mysteriously weaken and dissipate as they get to us, during the warm time of year.
When the winds blow from the northwest to Athens and NE Georgia, they go downhill. The downward motion inhibits the rising motion you need for clouds and storms.
See the attached satellite image of Georgia, from 1907Z = a little after 2 pm today. There are clouds all over north Georgia, except for northeast Georgia. (The isobars let you know that the winds within a mile or so above the surface are generally from the northwest.) This is the downsloping effect. In wintertime, if we have snow in northwest winds just behind a low-pressure system, this is why the Atlanta suburbs may get some flurries or snow when Athens doesn’t. In summertime, similarly, Athens can miss out on thunderstorms in northwest flow while locations to the west that experience less downsloping get stronger storms.
This effect is easiest to see with a large cloud deck that covers a region, rather than a single thunderstorm line. Today is a good example; notice in the other satellite image (same time today, just a larger view) that there aren’t any clouds over South Carolina or North Carolina downwind of their upstream mountains, either. That, too, is downsloping.
Downsloping in Athens: it’s a real thing, and affects our weather whenever the winds are from the northwest. It’s one more illustration of the profound effect that the southern Appalachians have on our weather, even though we’re not *in* the mountains.